In the prior art, inexpensive plastic solar cells based on exciton (bound electron/hole pairs) physics have been fabricated with AM1.5 efficiency of 2-3% (Huynh et al., “Hybrid Nanorod-Polymer Solar Cells”, Science 295, 2425-2427 (2002); Schmidt-Mende et al, “Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics”, Science 293, 1119-1122 (2002). Shaheen et al. “2.5 Percent Efficient Organic Plastic Solar Cells,” Applied Physics Letters 78, 841-843 (2001).
In these solar cell devices, however, the device architectures are suboptimal in terms of their nanometer-scale morphology, and improvements are needed to obtain higher-efficiency devices. In particular, the morphology of the active layer of a more optimal device would have an architecture of nanometer scale ordered interdigitation due to the nature of the exciton-based physics. The lifetime of migrating excitons is extremely short, and as such an exciton can typically diffuse only about 10 nm (or at most 10's of nm) before the electron and hole spontaneously (and non-productively) recombine. Thus, to separate the electron away from the hole with which it is bound (and ultimately generate electricity), an exciton must reach the junction to another material (one with higher electron affinity) within 10's of nm of where it was initially created. In solar-cell devices of the prior art, the morphology of the active layer has been quasi-random. For instance, in the Huynh et al. work, the nanorods are sprinkled across the polymer in which they are embedded; they often cluster into clumps of nanorods, producing some areas with more nanorods than ideal and other areas with fewer nanorods than ideal; and not all nanorods are fully connected so that islands may exists where charges are trapped (and even if full connectivity exists, charges generally cannot move out of the cell in a simple straight path, with interim recombination losses resulting). In the polymer-blend work of Schmidt-Mende and others, phase separation leads to a morphology of the active layer that is not well controlled, with islands and percolation paths that are not straight.
Finally, in devices of the prior art, the movement of charges through the active materials of the devices required regularly and closely spaced nanoparticles or nanorods which could collect and transport free electrons to the outer boundary of the active layer of the device, and the lack of uniform spacing in these devices decreased the hole and electron transport efficiency. All of these factors combine to reduce the device efficiency, and therefore the potential electricity that can be produced by a solar cell. Finally, devices of the prior art link materials choice with architecture, making it hard to optimize materials independent of shape.
Thus, there is a need in the art for a solar cell architecture/active-layer morphology that overcomes the above difficulties.